Originally Posted by poppalarge
That's the one.
How does it work?
A refractometer measures the sugar content of the wort by measuring how far light bends (or refracts) when as it passes through. You can only take direct measurements prior to fermentation, though. Once you have alcohol in the mix, you need to use a formula to correct the readings. Since you're unlikely to miss your OG when extract brewing, you're better off just using your hydrometer. However, they come in really handy when/if you go all-grain since you'll want to take multiple samples of hot wort, and the few drops you'll need will cool really quickly.
By the way, if you search ebay, you can find them for much less than $100. More like 30 bucks. Just make sure you get one with Automatic Temperature Correction (ATC) as they don't cost much more and it's one less calibration you have to keep track of.